In 1882, on the basis of an Otto cycle internal combustion engine, British engineer James Atkinson designed a complicated linkage mechanism, which allowed for a longer expansion stroke compared to the compression stroke. This clever design not only improves the intake efficiency of the engine, but also led to a higher expansion ratio compared to the compression ratio, which effectively improves engine efficiency; the working principle of such an engine is referred to as the Atkinson cycle. However, with the Atkinson cycle, the intake flowrate of the cylinder is small and the torque performance is poor when the engine is at low speeds, whereas long piston strokes are not conducive to a high-speed operation of the engine. The Atkinson cycle can effectively exert its power only at intermediate speeds. Therefore, in the past, when dynamic performance was pursued, researches on Atkinson cycle engines were ignored.
In recent years, the development of automotive hybrid power technology has brought the Atkinson cycle back to attention. The wheels of a hybrid vehicle are powered by an electric motor at either low or high speeds, and the engine is generating electricity most of the time; therefore, the engine can run at a speed that allows the most economical fuel consumption. This is exactly where the advantage of high thermal efficiency of the Atkinson cycle can be fully exploited. Therefore, under the pressure of fuel consumption regulations, domestic and overseas automobile companies began to study the Atkinson cycle. It can be said that the Atkinson cycle is one of the key technologies of hybrid vehicles.
The calculation of intake flowrate is a key issue in Atkinson cycle control. This is because in engine control, the adjustment of parameters, including engine output torque, fuel injection amount, ignition angle, and throttle opening, is based on accurate calculation of the intake flowrate. At present, there are two main methods for calculating the intake flowrate of an internal combustion engine in real vehicles: one method is based on an air intake mass flow sensor: the amount of fresh gas entered into the engine cylinder is calculated from mass flow signals detected by the air intake mass flow sensor. However, under some working conditions of the Atkinson cycle, high-intensity backflow occurs, leading to mass flow signal deviation that results in inaccurate calculations. The other method is based on an intake pressure sensor: in a conventional engine, intake valve closure occurs when the piston is near the intake bottom dead center; at this point, the pressure within the cylinder is approximately equal to the intake pressure. The state within the cylinder at intake valve closure can be therefore obtained, and the quantity of fresh gas entering the cylinder can be calculated. However, in an Atkinson cycle, the position of the piston at intake valve closure is far from the position of the piston at the intake bottom dead center, and the pressure within the cylinder is considerably different from the intake pressure. Therefore, this method is not applicable to an Atkinson cycle machine.